As the world's need for communication capacity continues to increase, the use of optical signals to transfer large amounts of information has become increasingly favored over other schemes such as those using twisted copper wires, coaxial cables, or microwave links. Optical communication systems use optical signals to carry information at high speeds over an optical path such as an optical fiber. Optical fiber communication systems are generally immune to electromagnetic interference effects, unlike the other schemes listed above. Furthermore, the silica glass fibers used in fiber optic communication systems are lightweight, comparatively low cost, and are capable of very high-bandwidth operation.
Optical amplifiers are important components of optical communications links. Optical amplifiers are commonly used as (i) power amplifiers at the source end of an optical communications link, (ii) line amplifiers along the optical signal transmission path, and (iii) preamplifiers at the receiving end of the optical communications link, and have other uses as well.
In general, the two primary types of optical amplifiers are optical fiber based amplifiers, such as erbium doped fiber amplifiers (EDFAs) and Raman amplifiers, and semiconductor optical amplifiers (SOAs). EDFAs are widely used in line amplifiers and other applications requiring high output power, high data rates, and low noise. However, EDFAs are quite bulky, having a typical fiber length of about 30 feet, and require the presence of a separate pumping laser to operate. Accordingly, EDFAs are difficult to incorporate into confined spaces, and are certainly not amenable to circuit-board-level or chip-level integration.
SOAs, on the other hand, are small in size and conveniently integrated into small devices. An SOA generally resembles a semiconductor laser structure, except that the end mirrors are replaced by antireflection coatings. In such devices the product of the gain and the reflectivity is less than one so that the device does not oscillate. Rather, the device is used to amplify an incoming optical signal as it passes through the device. Such devices are often called traveling wave amplifiers, which highlights the fact that the optical signal does not pass back and forth within the device, but merely passes through it essentially only once. SOAs generally yield lower output power and higher noise levels as compared to EDFAs, and/or are restricted to lower data rates at high output power due to pattern dependent amplitude variations. Research continues toward improving the performance of SOAs, including making SOAs with higher power and lower noise characteristics, and/or that are capable of operating at higher data rates at high input power without suffering significant pattern dependent amplitude fluctuations.
Crosstalk is one of the primary troublesome noise sources in conventional SOAs. Crosstalk, or cross-channel modulation, involves data-dependent gain fluctuations at high output levels from the SOA, and can occur for either time-multiplexed or wavelength-multiplexed data. Crosstalk arises from gain saturation effects in an SOA. These effects can be understood by recalling that SOA devices rely on the phenomenon of stimulated emission to provide the necessary amplification. Stimulated emission, in turn, requires the establishment of a population inversion. In typical SOAs or lasers a population inversion is evidenced by the presence of a specified carrier density. When a sufficiently large optical signal is passed through the amplifier, the population inversion is substantially reduced or depleted, i.e. the gain of the SOA is saturated, and is reestablished only over some finite period of time. Consequently, the gain of the SOA will be reduced for some period of time following the passage of the signal through the amplifier, a time period commonly denoted as the amplifier gain recovery time.
When the gain medium becomes saturated due to a high signal level on a first channel, changes are induced in the signal level of a second channel because the saturated gain medium cannot properly amplify both channels. Since gain is modulated by the first signal, this modulated gain is impressed on the second signal. Thus, for wavelength division multiplexed (WDM) systems in which a plurality of channels at λ1,λ2, . . . , λN are present in a common optical signal, gain saturation induced by a first channel at λ1 can produce unwanted level changes (i.e., errors) in a second data channel at λ2, and vice versa.
Crosstalk can be reduced by keeping the SOA out of gain saturation for the data rates, signal levels, and number of channels on the optical signal of interest. If the SOA is operated near gain saturation levels, crosstalk may be reduced by making the “off” portion of the signal duty cycle longer in comparison to the gain recovery time, i.e., by slowing down the data rate. In general, an SOA will have reduced crosstalk effects if (i) its saturation power PSAT, i.e., the input optical power level for which the SOA gain is reduced to a predetermined percentage of its nominal value, is increased, and/or (ii) its gain recovery time is decreased. As used herein, an SOA has increased gain stability if (i) its saturation power PSAT is increased without a concomitant increase in gain recovery time, (ii) its gain recovery time is decreased without a concomitant decrease in saturation power PSAT, or (iii) both (i) and (ii) occur.
Several methods for dealing with crosstalk problems are discussed in U.S. Pat. No. 5,436,759, which is incorporated by reference herein. One particularly appealing strategy is to place a transverse laser across the SOA such that the laser's gain medium and the SOA's signal gain medium share an overlapping region. The lasing cavity is operated above threshold and the gain of the laser is clamped to overcome the losses of the cavity. As used herein, a laser cavity is gain-clamped and lasing when it is excited by a bias current greater than a threshold current. When the transverse laser is gain-clamped, gain along the SOA signal path is stabilized. The transverse lasing enhances the establishment and maintenance of a population inversion in the overlapping region, resulting in both increased saturation power and a decreased gain recovery time due to increased photon density in the laser cavity. Advantageously, independent lasing only builds up in the transverse direction and does not corrupt the quality of the amplified signal. Other methods in which the gain media of SOAs is coextensive with the gain media of transverse lasers are discussed in Francis, et. al., “A Single Chip Linear Optical Amplifier,” IEEE Optical Fiber Communication Conference, Anaheim, Calif. (2001), which is incorporated by reference herein, and in U.S. Ser. No. 09/972,146, supra.
Amplified spontaneous emission (ASE) is another primary troublesome noise source in conventional SOAs. ASE arises from random, spontaneous energy state drops in a small fraction of the excited carriers of the gain medium. Light emitted as a result of these energy state drops is generally random in direction and wavelength. Some of this light will be emitted in the direction of signal propagation and will therefore be amplified as it propagates, resulting in output background noise signal similar to white noise. For good performance, it is generally desirable to keep the amplified signal levels at least 10 dB higher than ASE levels.
One potential disadvantage of an SOA having a gain medium coextensive with the gain medium of a transverse laser cavity is that ASE levels in the output can be relatively high. This is because the population inversion of the gain medium, and therefore the number of carriers experiencing spontaneous energy state drops, is maintained at high levels by the transverse lasing field. Other potential drawbacks, include polarization sensitivity, output power limitations, and reduced coupling efficiency are related to the generally thin nature of the gain medium layer itself in such devices. Whereas the width of the signal path is usually on the order of 8 μm for single-mode propagation, the height of the signal path is generally limited to the thickness of the gain medium layer, which is often less than 1 μm. Thus, the signal waveguide has a small cross-section (as compared to single-mode waveguides having larger aspect ratios), which limits overall power carrying capacity and reduces coupling efficiency with external planar lightwave circuits and optical fibers. Moreover, the small height dimension causes the signal waveguide to prefer one light polarization over another, causing the SOA to be polarization sensitive and difficult to couple to optical fibers.
The proposed designs discussed in U.S. Pat. No. 5,436,759, supra, also suffer from other shortcomings that can reduce the effectiveness or usefulness of the SOA device and/or cause difficulty in reliably fabricating the device. For example, while the electrical pumping current applied to the laser cavity dictates the presence of gain-clamped operation (below-threshold current yields unclamped operation, above-threshold yields clamped operation), there are no controls provided that allow for dynamic control of the actual amount of gain once the gain is clamped. As another example, the layers of the single active medium ('759 patent, FIG. 1) can be difficult to construct reliably with consistent thickness across the entire lateral area to be covered. In addition, the semiconductor layers may contain local defects such as crystal dislocations, pitting, voids, etc. Such defects in the epitaxial growth can be a point of lower electrical resistance than the surrounding epitaxial areas. The higher electrical current flowing through these points of lower electrical resistance can create “hot spots” which cause non-uniform gain in the effected areas. In addition to the less than optimal performance resulting form the non-uniform gain, the “hot spots” can be a source of excessive current drain and premature device failure. As another example, undesired parasitic modes or uneven lasing may arise due to lack of electrical isolation among laser cavity segments at intersections with the signal gain medium.
The approach discussed in Francis, supra, in which the signal waveguide and a vertical cavity surface emitting laser (VCSEL) share the same active region, can also suffer from non-uniform current flows and hot spots due to defects in crystal growth and non-uniform epitaxial layers. Moreover, such devices can have limited output power due to the small gain medium volume of VCSELs, as well as for other reasons.
U.S. Pat. No. 5,291,328, which is incorporated by reference herein, discusses a semiconductor optical amplifier having an active layer vertically separated from a signal waveguiding layer in order to reduce polarization sensitivity. The active layer and passive waveguide layer are vertically collinear between the input and output facets of the device. According to the discussion of the '328 patent, by using a structure which is analogous to a buried-heterostructure distributed feedback laser having a waveguiding layer and an active layer in close association, but without the grating, it is possible to achieve high gain and low polarization sensitivity over a usefully broad wavelength range. However, the device discussed in the '328 patent is not designed for lasing action to occur in the active layer, having antireflective coatings at the end facets, and would accordingly suffer from the crosstalk problems discussed supra. Moreover, the device can also suffer from non-uniform current flows and hot spots due to defects in crystal growth and non-uniform epitaxial layers, although such problems are less relevant because the device does not achieve lasing action anyway.
U.S. Pat. No. 4,742,307, which is incorporated by reference herein, also discusses a semiconductor optical amplifier having an active layer vertically separated from a passive waveguide layer, for the discussed purpose of reducing ASE noise in the output. The active layer and passive waveguide layers are vertically collinear between the input and output facets of the device. Notably, the device of the '307 discussion has increased separation between the active layer and the passive waveguide layer near the ends of the device for easier coupling between external fibers and the signal path ('307 patent, FIGS. 1A, 1B, 7). However, this separation is in the vertical direction, and must therefore be achieved by differential growth processes or other more complicated fabrication steps. This is particularly troublesome where appreciable separations approaching 8 μm or more are needed for efficient coupling into fibers. Moreover, as with the device of the '328 patent, the device of the '307 patent is not designed for lasing action to occur in the active layer, and would accordingly suffer from the crosstalk problems discussed supra. The device can also suffer from non-uniform current flows and hot spots due to defects in crystal growth and non-uniform epitaxial layers although, as with the '328 device, these problems are less relevant because the device does not achieve lasing action anyway.
Accordingly, it would be desirable to provide a semiconductor optical amplifier (SOA) having reduced crosstalk effects while also having reduced polarization sensitivity.
It would be further desirable to provide an SOA having reduced crosstalk effects while also having reduced amplified spontaneous emission (ASE) noise levels.
It would be even further desirable to provide an SOA having reduced crosstalk effects while also allowing dynamic adjustment of gain during gain-clamped operation.
It would be still further desirable to provide such an SOA capable of achieving a variable gain characteristic along its length.
It would be even further desirable to provide such an SOA in which signal power capacity is increased and coupling efficiency with optical fibers and planar lightwave circuits is increased.
It would be even further desirable to provide such an SOA that can be reliably fabricated.
It would be still further desirable to provide such an SOA that is more operationally robust against the presence of local defects that may occur during device fabrication.